This invention is generally related to micro-electromechanical (MEM) switches and to a method of fabricating such structures, and more specifically, to a MEM variable capacitor that uses three-dimensional comb-drive electrodes (fins) by bonding a chip to a carrier substrate.
Variable capacitors or varactors are a fundamental part of high-frequency and radio-frequency (RF) circuits. MEM variable capacitors have drawn considerable interest over the last few years due to their superior electrical characteristics. Variable capacitors using MEM technology can be easily implemented in standard semiconductor devices for applications in aerospace, consumer electronics and communications systems.
Many researchers have attempted to improve the tuning range of MEMS variable capacitors since the maximum capacitance tuning range achieved by using a parallel plate electrode approach is limited. This is due to the non-linear electrostatic forces involved during actuation. The parallel plate electrodes exhibit a typical xe2x80x9cpull-down behaviorxe2x80x9d at one-third the gap distance, leading to a maximum tuning capacitance of 1.5. Most previous approaches have resulted in increased processing complexity and/or a large number of moving parts, leading to a drastic reduction in reliability. Additionally, packaging the MEMS device and integrating it into CMOS integrated circuit pose great challenges.
A. Dec et al., in an article entitled xe2x80x9cRF micro-machined varactors with wide tuning rangexe2x80x9d, published in the IEEE RF IC Symposium Digest, pp. 309-312, June 1998 describe building a MEMS variable capacitor by actuating the movable electrode using two parallel electrodes above and below the movable electrode. The total capacitance tuning range is significantly enhanced as a result of the individual capacitance between the top-movable and movable-bottom being in series. The maximum tuning range achievable using this approach is a ratio of 2:1. A. Dec et al. have reported achieving a tuning range as high as 1.9:1. Even though the tuning range is significantly improved using this approach, the process complexity is increased.
The inherent electromechanical aspects involved in present approach are quite different than the parallel plate approach. Comb-drive electrodes are used for actuation while control or signal electrodes sense the motion of the movable electrode. The resulting capacitance tuning range is greatly enhanced since the electrostatic forces are constant in nature. Since this device has three ports (two ports for DC bias and one port for the RF signals), the signal capacitance requires decoupling as is the case in a 2-port varactor device. Moreover, most prior art MEMS devices need to be separately packaged that, at least for MEM devices with moving parts creates certain processing issues that need to be resolved.
Accordingly, it is an object of the invention to provide a MEM variable capacitor device that utilized comb-drive electrodes (or fins) for actuation, while the control or signal electrodes sense the motion of the movable beam, leading to a change in capacitance.
It is another object to provide a MEM varactor device where the switch contacts are separated by a dielectric to provide electrical isolation between the control signal and the switching signal.
It is further an object to provide a MEM varactor device with comb-drive actuation for obtaining large capacitance ratio or tuning range.
It is yet another object to configure a plurality of MEM varactor devices in a variety of three dimensional arrangements.
It is still another object to provide a MEM varactor with increased drive electrode area for lower drive voltages.
It is still a further object to provide a method of fabricating a MEM switch using manufacturing techniques that are compatible with those applicable to CMOS semiconductor devices, which allows fabricating and packaging the MEMS device simultaneously and reduces the number of fabrication steps to a minimum while reducing the cost of processing.
MEMS based variable capacitors provide many advantages over conventional solid-state varactors. These devices are operated at higher quality factors leading to low loss during operation. Two types of MEMS varactors are described herein: parallel plate and comb-drive varactors. Most widely investigated MEMS varactors are parallel plate capacitors with a movable electrode and a fixed electrode. The major disadvantage when using these MEMS devices is the limited tuning range of operation obtained upon actuation of these devices. The inherent electromechanical aspects involved restrict the tuning range and lead to snap down of the movable electrode. This is often stated as xe2x80x9cpull-down instability effectxe2x80x9d. The electrostatic forces acting on the movable electrode are non-linear in nature which cause this effect. In the comb-drive electrodes, the electrostatic forces acting on the movable electrode are linear in nature (directly proportional to the distance) which greatly enhances the tuning range. However, comb-drive electrodes are difficult to process and the change in capacitance obtained is very small (due to less area available).
In one aspect of the invention, the MEM switch described includes both of the approaches (parallel plate and comb-drive) that were thus far considered. Greater area is made available during tuning by using a parallel plate type model while incorporating the linear electrostatic forces from the comb-drive approach. The movable and fixed electrodes are processed separately on chip and carrier wafers. The chip side contains the fixed-fixed movable beam. The beam is fabricated with metal xe2x80x9cfinsxe2x80x9d acting as comb electrodes. The carrier side has an actuator (DC electrodes) along the side walls and bottom of the trenches. The RF (signal) electrodes are positioned between the electrodes. The actuator electrodes are connected by way of xe2x80x9cthrough viasxe2x80x9d for electrical connection.
After completion of processing both the chip and the carrier wafers, the chip side is flipped onto the carrier wafer and precision aligned so as to make electrical connection. The height of the stud on the carrier side determines the air gap between the movable electrode and the fixed electrode. Finally the device can be encapsulated with polymeric material in order to provide controllable environment for the MEMS device during operation.
In another aspect of the invention, there is provided a semiconductor micro electro-mechanical (MEM) varactor that includes a first substrate having a movable beam anchored at least at one end of the movable beam to the first substrate, the movable beam having discrete fins protruding therefrom in a direction opposite to the first substrate; and a second substrate coupled to the first substrate having fixed electrodes, each of the fixed electrodes respectively facing one of the discrete fins, the discrete fins being activated by a voltage between the protruding fins and the fixed electrodes.